Last summer, a network
of buoys straddling the equator in the eastern Pacific registered abnormally high
sea-surface temperatures  signaling that El Niño, a driver of global
climate, had resurfaced after a four-year hiatus. The warm pool of water triggered
changes in jet streams that brought unseasonable warmth to the northern United
States west of the Rockies this winter, and a deluge of moisture to drought-ridden
states along the Gulf Coast.

Scientists from the National Oceanic and
Atmospheric Administration adjust sensors on a buoy in the equatorial Pacific.
A network of buoys spanning from the coast of Ecuador westward to Indonesia record
ocean temperatures and winds  data critical for predicting and monitoring
El Niño and La Niña events. Photo courtesy of NOAA/Department of
Commerce.

While forecasters can now predict El Niño events up to a year before they
reach their peak, the impacts of long-term climate change on El Niño remain
difficult to pin down. Some climate models suggest that global warming could lock
the climate into a permanent state of El Niño or into its counterpart La
Niña, which typically brings storms to the northern United States in the
winter. Gone would be the cyclic alternation between El Niño and La Niña,
and the associated cycle in atmospheric pressure called the Southern Oscillation
 known collectively as ENSO  that define modern climate.

A paper in the Feb. 7 Science looks back to the Eocene, a time 55 to 35
million years ago when the global climate was 10 degrees Celsius warmer than it
is now, and found evidence for ENSO cycles similar to those today. Past robustness
suggests that ENSO will not break down under future warming, the authors say.

The study fundamentally challenges some of the papers that have suggested
that the future of El Niño is dire in a greenhouse-warmed climate,
says geologist Donald Rodbell of Union College in Schenectady, N.Y.

The researchers adapted the Community Climate System Model of the National Center
for Atmospheric Research to match Eocene conditions. The publicly available model
simulates ocean, land and atmosphere interactions. Going back to the Eocene meant
tailoring the model by shifting plate locations, changing ocean basin morphologies
and doubling the atmospheric concentrations of the greenhouse gas carbon dioxide.

The Eocene provides a particularly exacting test of the ENSO shutdown idea,
says co-author Rodrigo Caballero, an atmospheric scientist at the University of
Chicago. It features the warmest temperatures of the past 60 million years.
If ENSO didnt shut down then, it becomes very difficult to argue
that it has shut down at any later time.

Theories predicting a permanent El Niño suggest that global warming will
disrupt a delicate feedback between the ocean and the atmosphere, says co-author
Matthew Huber, a climatologist at Purdue University. Today, an El Niño
event ends when trade winds blowing from east to west across the Pacific pick
up speed. The winds push the warm pool of water off the coast of Ecuador to the
west. Cold water from below upwells, replacing the departed warm water. Temperature
differences between the eastern and western Pacific, in turn, strengthen the trade
winds. That positive feedback draws up more cold water in the east that eventually
dissipates the anomalous surface water warmth that defines El Niño. A greenhouse
atmosphere could raise the temperature of the upwelling water, which would undermine
the east-west temperature gradient that drives the feedback that pulls the climate
out of El Niño.

In the Eocene model, atmospheric warming does raise the temperature of the oceans.
However, the majority of the added warmth gets locked into highly saline water
that sinks to the bottom of the ocean. The top kilometer of ocean water remains
relatively cool. Because trade winds pull water up from several hundred meters
below the surface  not from the very deep ocean water  the model sustains
the cold-water upwelling necessary to break El Niño events. In the simulations,
El Niño develops and disappears once every three to five years, as observed
today.

Sedimentary records from two lakes appear to support the simulations: one from
Wyoming, reported by Maurizio Ripepe from the University of Camerino, Italy; the
other from Germany, reported by J Mingran, from GeoforschungsZentrum in Potsdam,
Germany. Annual bands of sediment laid down during the Eocene suggest that floods
and heat waves pulsed through those regions at roughly the frequencies predicted
by the Eocene model.

However, both the model and the field data must be taken with more than a grain
of salt, says Princeton geophysicist George Philander. The Earth climate
has strong asymmetry relative to the equator, and most models fail to reproduce
it. They cant really get El Niño [today] correct. Until
the simulations model the variability of todays climate more accurately,
back calculations of variability in the distant past remain questionable, Philander
says. And the lake sediments were so far away from the equatorial Pacific during
the Eocene that other climate variables likely masked any true El Niño
signals. The last place you would go to look for El Niño signals
today would be Germany, says Philander.

Yet very few annual records from the Eocene exist, Caballero says. One of
the aims of our paper was to generate sufficient interest to stimulate the data
people to actually go out and search for such records. An Eocene record
obtained directly from the tropics  from sediments in South America, for
example  would provide an ideal test for the model, he adds.